1. Field of the Invention
The present invention relates to a gain-coupling type distributed feedback semiconductor laser device, and a method for producing the same.
2. Description of the Related Art
Distributed feedback (DFB) semiconductor laser devices are utilized as coherent light sources having variable or stabilized wavelengths and capable of operating in a monoaxial mode, and as such are increasingly in demand in the field of optical measurement, optical communications/transfer, optical recording, laser beam printers, and the like. In a DFB laser device, a diffraction grating is provided in an active layer or a guide layer for causing optical feedback so as to provide laser oscillation. DFB laser devices are generally classified into two categories: refractive index-coupling (IC) DFB laser devices, in which distributed feedback is attained by periodically varying the refractive index provided by a diffraction grating; and gain-coupling (GC) DFB laser devices, in which distributed feedback is attained by periodically varying the gain provided by a diffraction grating.
Due to their operation principles, IC-DFB laser devices are likely to laser oscillate in two modes because a stop band which exists in the vicinity of the Bragg wavelength is interposed between two symmetrically-positioned longitudinal modes having minimum threshold gains. On the other hand, GC-DFB laser devices have been theoretically confirmed to yield a single longitudinal mode with a probability of substantially 100%, irrespective of the end face phase.
GC-DFB laser devices can be further classified into two subcategories, with respect to the kind of structure that is employed to provide a periodically varying gain: a gain diffraction grating structure, in which the active layer provides a periodical varying gain; and an absorptive diffraction grating structure, which incorporate periodic regions for absorbing part of the induced emission that is generated from the active layer, thereby effectively providing a periodically varying gain.
Since the absorptive diffraction grating includes regions which are periodically located in the vicinity of the active layer for absorbing part of the induced emission generated from the active layer, a large absorption loss of energy may result, thereby increasing the oscillation threshold. However, the gain diffraction grating structure is free from the problem of increased oscillation thresholds because it does not incorporate any regions for absorbing a portion of the induced emission generated from the active layer.
Some conventional DFB laser devices incorporate a quantum well structure in their active layers. A quantum well structure features light-emitting layers which are composed of very thin well layers (e.g., several nanometers), with a view to enhancing the carrier density within the active layer and improving the laser characteristics of the device.
FIG. 8 is a cross-sectional view illustrating a conventional gain diffraction grating type GC-DFB laser device incorporating a quantum well layer structure as an active layer (taken along the direction of its laser cavity).
This conventional GC-DFB laser device is constructed as follows: First, an n-cladding layer 81 and an undoped quantum well layer 82 are formed on an n-substrate 80 by MOCVD (metal organic chemical vapor deposition). The undoped quantum well layer 82 includes well layers 82a and barrier layers 82b. Next, a diffraction grating is impressed on the uppermost layer among the growth layers by a two-beam interference exposition method and etching. Then, a further growth (regrowth) process is performed by MOCVD to form a p-cladding layer 83, and p-contact layer 84. Thereafter, electrodes 851 and 852 are deposited. Finally, the wafer is cleaved, whereby the semiconductor laser device is completed.
This conventional laser device provides a periodic gain distribution by directly etching the active layer, thereby realizing a GC-DFB laser device.
However, in accordance with the above conventional example, it is necessary to first perform a growth process up to the active layer, temporarily suspend the growth to allow the impression of a diffraction grating thereupon, and then resume the growth (regrowth) process to further form a cladding layer. Generally, during the period of suspended growth which proceeds a regrowth process, the substrate surface is exposed to the atmosphere and therefore degraded; as a result, the crystallinity of the material at the regrowth interface is greatly deteriorated. In the case of the above conventional example, in which a relatively large area within the active layer defines a regrowth interface, the crystallinity at the regrowth interface within the active layer undergoes severe deterioration. As a result, the semiconductor crystalline structure of the active layer suffers from defects.
Such crystal defects increase a recombination component which is unrelated to light emission, thereby greatly reducing the induced emission. This degrades the characteristics of the semiconductor laser and greatly reduces the reliability of the device.